In next-generation radio communication systems, there are concerns over exhaustion of frequency resources as the transmission rates increase with wider bandwidths and systems are increasingly diversified. In such circumstances, cognitive radios have been studied that identify surrounding radio wave environments and users' needs to autonomously optimize communication parameters. Dynamic spectrum access, which allows a frequency band assigned to an existing radio communication system to be secondarily used by another radio communication system, has in particular drawn attentions in terms of the efficient use of frequency resources.
Referring to FIG. 1, the dynamic spectrum access is a technology in which a spectrum available in a frequency band that is assigned to a first radio system 10 is used by a new second radio system 11 in such a manner that the second radio system 11 will not interfere with communication in the first radio system 10. FIG. 1 depicts, as an example, a case where an uplink of the second radio system 11 shares a frequency band assigned to a downlink of the first radio system 10. That is, in the first radio system 10, a transmitter 12 sends data to a receiver 13, while in the second radio system 11, a transmitter 14 uses the same frequency band to send data to a receiver 15. In principle, the transmitter 14 of the second radio system performs communication by using, for example, transmission power control so that interference with the receiver 13 of the first radio system 10 will not be higher than a defined value because when a frequency is shared, communication in the second radio system 11 need to be performed in such a manner as not to interfere with communication in the first radio system 10. Thus, it is possible to suppress interference from the second radio system 11 to the first radio system 10.
On the other hand, the receiver 15 of the second radio system 11 receives its own system's desired waves 21 transmitted from the transmitter 14 and, at the same time, receives a signal arriving from the transmitter 12 of the first radio system as interfering waves 20. Accordingly, at the receiver 15 of the second radio system, transmission characteristics deteriorate due to interference arriving from the transmitter 12 of the first radio system. If the transmitter 14 of the second radio system 11 is a mobile station such as a mobile communication terminal and its transmission frequency shares the frequency band of the first radio system 10 in particular, the separation distance between the first radio system 10 and the second radio system 11 is short because the transmission power of the transmitter 14 is small and the antenna height thereof is low, and consequently the interfering waves 20 arriving from the transmitter 12 of the first radio system have a great influence.
Various technologies for suppressing the influence of interfering waves have been proposed. For example, according to a radio receiver with multiple antennas disclosed in NPL 1, the channel estimate values of desired waves 21 and the channel estimate values of interfering waves 20 are determined, and based on them, an equalization weight is generated that minimizes the square error between a pilot code and an equalized signal, thereby suppressing interference. Moreover, PTL 1 also discloses a communication device that suppresses the influence of interfering waves by estimating the channel characteristics of interfering waves and desired waves by using a pilot subcarrier.